Numerous applications exist in agriculture for sensors capable of detecting characteristics of objects present in the field. One example of such an application, which is presented only for illustrative purposes here, involves orchard management. In order to control pests, chemical insecticides are applied to orchard trees using a sprayer attached to a vehicle such a tractor. Due to the high cost of such insecticides and due to the potential adverse impact such insecticides may have on the environment, it is desirable to minimize the use of such insecticides. If individual trees in the rows of an orchard could be discriminated from the spaces between the trees, then a sprayer proceeding through the orchard between two rows of trees could be controlled to apply insecticide onto the trees and not to waste insecticide by spraying insecticide into the spaces between trees.
In an attempt to meet this need, sonar technology has been used. When a sonar transducer of a sonar-controlled orchard sprayer senses that a tree is in close proximity to the sprayer, the sprayer is energized and insecticide is sprayed onto the tree. Otherwise, the sprayer is not energized and the insecticide that otherwise would have been sprayed into the spaces between trees is preserved.
This sonar technology, however, has fundamental limitations. Sonar sprayers often fail to discriminate between sonar reflections off objects and ambient acoustic interference. Accordingly, it is common for sonar-controlled sprayers to energize when no tree is in fact present. Moreover, sonar-controlled sprayers are typically unable to discriminate between living trees and other objects such a buildings, vehicles, power poles and people. The sprayer may therefore waste insecticide by spraying non-tree objects like barns. Such systems also present a safety hazard because a sonar-controlled sprayer may mistake a person for a tree and spray the person with insecticide. A need therefore exists for a proximity sensor which is capable of discriminating trees from other objects which should not be sprayed.